We wish to understand, on a molecular level, enzyme catalysis in which molecular oxygen is cleaved and incorporated into organic substrates. These reactions represent the major route of incorporation of oxygen into cellular molecules and as such perform essential metabolic functions in both prokaryotic and eukaryotic organisms. Moreover, the oxygenase enzymes actively metabolize molecules which impact on both human health and the world environment. We have chosen for study a representative group of iron containing di- and mono-oxygenases which include protocatechuate dioxygenases, catechol dioxygenases and cytochrome P450. We are using the systematic application of diverse techniques to describe mechanisms at every level of protein structure; these include: optical, ENDOR, EPR, NMR and Mossbauer spectroscopy; chemical modification, amino acid sequence determination and synthesis of transition state analogs; low temperature, steady state and presteady state kinetics. Our goals are 1) to formulate reasonable and defensible mechanisms for oxygenases at the molecular level, 2) to identify the form of oxygen which reacts with substrates, 3) to understand the role of iron in these metalloproteins, 4) to understand the bearing on catalysis of protein interactions with other proteins, solvent and membranes.